1. Field of the Invention
The present invention relates to the generation of tunable coherent radiation, and more specifically, it relates to a frequency agile parametric wavelength converter that can be used for applications in, e.g., spectroscopy, remote sensing, and laser radar.
2. Description of Related Art
U.S. Pat. No. 5,841,570 titled xe2x80x9c Frequency Agile Optical Parametric Oscillator,xe2x80x9d discloses a design for a frequency agile optical parametric oscillator (FA-OPO) that permits pulse-to-pulse changes in the output frequency at repetition rates limited only by the pulsed laser repetition rate and the angle deflection rate of commercial beam deflector technology. The design disclosed in the referenced patent has the following three limitations:
A first limitation of the previous design arises because it is based on the principle of birefringence phasematching, which permits the existence of a nearly linear relationship between the angle of the pump beam, about the collinear phase matching condition in the nonlinear crystal, and the wavelength generated. Thus, a useful range of output wavelengths can be generated with small changes in pump beam angle, making the device practical. However, obtaining a desired tuning range depends on finding a particular crystal that will permit collinear birefringence phasematching at the center wavelength of the desired tuning range. This feature restricts the applicability of the frequency agile optical parametric oscillator design to particular wavelength regions determined by available crystals. Moreover, because the effective nonlinear coefficient and walk-off angle in the available crystal cannot be controlled, restrictions on the required minimum input pump pulse energy for efficient OPO wavelength conversion may apply.
A second limitation in the previous design occurs because, as the pump beam angle is changed, there is a corresponding rapid change in the overlap between the pumped volume in the nonlinear crystal and the mode volume of the resonated wave, leading to distortions of the output beam shape and a reduction of the output energy.
A third limitation in the previous design is that the relay telescope component does not permit a compact folded path to reduce the physical xe2x80x9cfoot printxe2x80x9d of the device.
It is known that quasiphasematched crystals such as periodically poled lithium niobate (PPLN) can be fabricated to achieve collinear phasematching to produce any desired wavelength from an OPO by changing the quasiphasematching period. In addition, quasiphasematched crystals are generally fabricated in a way that achieves very large nonlinear coupling coefficients, which leads to larger frequency conversion efficiencies. However, quasiphasematched crystals have very small angle tuning ranges around their collinear phasematching orientations, and to-date, large changes in output wavelength could only be achieved by using very large pump angles. At large angles, the overlap of the interacting light waves is reduced, lowering the efficiency of the device. The OPO design of the present invention allows the practical use of quasiphasematched crystals in a frequency agile OPO by greatly increasing the tuning rate and reducing the change in overlap over the same tuning range.
It is an object of the present invention to provide an improved frequency agile optical parametric oscillator that includes a quasiphasematched crystal (QPMC) with a tilted grating, an off-axis reflective relay telescope, and an anamorphic beam expander.
This device improves upon a previously disclosed design U.S. Pat. No. 5,841,570 titled xe2x80x9cFrequency Agile Optical Parametric Oscillator,xe2x80x9d incorporated herein by reference, by (1) permitting a more flexible class of nonlinear crystals to be used, (2) providing more efficient operation over a wider tuning range, and (3) permitting a more compact device to be fabricated.
In the prior art, obtaining a desired tuning range has depended on finding a particular crystal that will permit collinear birefringence phasematching at the center wavelength of the desired tuning range. In the FA-OPO of the present invention, this limitation is overcome by near on-axis pumping of a single QPMC with a tilted periodically poled grating.
The second limitation of the prior art design is overcome in the present invention by both the tilted grating design and by the elongation of the transverse profile of the pump beam in the angle-tuning plane of the FA-OPO. This reduces the rate of change of the overlap between the pumped volume in the crystal and the resonated and non-resonated wave mode volumes as the pump beam angle is changed. Tilting the domain period has the effect of shifting both the tuning curve symmetry point and the location of the peak effective crystal length. Since these two points shift at different rates, by tilting the grating a sufficient amount, the peak effective crystal length can be positioned to overlap with the desired tuning range.
In the prior art, the third limitation in the previous design is that the relay telescope component does not permit a compact folded path. This limitation is overcome in the present invention by using a folded mirror set to relay the pivot point for beam steering from the beam deflector to the center of the FA-OPO crystal. This reduces the footprint of the device by as much as a factor of two over that obtained when using the refractive telescope design cited in the previous invention.
On embodiment of the invention utilizes periodically poled lithium niobate (PPLN) as the QPMC crystal 24 material, and further includes OPO mirrors 20 and 22 that reflect light in the 2700-3300 cmxe2x88x921 region. This embodiment is pumped by an Nd:YAG laser with a wavelength of 1.06 xcexcm. FIG. 3 shows tuning curves for tilted and non-tilted grating PPLN OPO""s resonating light in the 2700-3300 cmxe2x88x921 region. The figure shows that the tuning range (cmxe2x88x921 per milliradian of pump angle) of the tilted grating device is improved over that of the standard non-tilted grating. This improvement permits substantially larger tuning ranges, thus making it practical to use a QPMC in an FA-OPO device. The performance of the FA-OPO is improved by the large nonlinear coupling coefficient of QPMCs and the flexibility of wavelength tuning range afforded by the variation in poling period. The availability of such a wide tuning range has implications for several lidar applications, including aerosol sizing and chemical detection.
The QPMC may comprise periodically poled lithium niobate, lithium tantalate, potassium titanyl phosphate (KTP), potassium titanyl arsenate (KTA) or rubidium titanyl arsenate (RTA). Quasiphasematched stacks of GaAs or related semiconductor structures may also be substituted for the QPMC.